Sandwich wall construction question...

[Elmosaurus]

Ok, so I'm getting tight on time. (no big surprise with the amount of things going on in my life)

I plan on using framed spar construction for my walls, and sandwiching them between 3/8" CDX for the exterior skin, and eventually some kind of 1/8" luan or sheeting inside. Cavities filled with 3/4" polystyrene, built using 1x2 Douglas Fir spars.

For those of you that have built tears using the sandwich wall construction: The thing I'd like to know if I can safely tow the unit if only the exterior skins are on, after I tie the two side walls together using 1x2 spars for the front, back, and roof. (ie, no interior skins, but entire frame is completed and exterior skins are all in place using all typical adhesive and screwing methods)

I know much of the strength of the wall comes from the completed sandwich construction of the walls. But I'm wondering, if the two walls are tied together, and the front, back, and roof are skinned, will that create the complete torsion box, even if the interior skins are not on?

Will the trailer be towable at reasonable highway speeds? or will it flex too much, and create enough stress along the joints that could crack a sealant/primer/paint coat?

If it's safe, it'll make life easier for me as I can get the unit sleepable for our deadline, and then I can take my time after we get back wiring and finishing the interior more nicely. (instead of pulling many all nighters during the final weekends)

Thoughts?

E.

[Miriam C.]

There are more than a couple of TD's built without interior skins and they do fine. Just be sure your bulkhead supports are in.

[aggie79]

I think you will be okay if you have a roof skin on and some sort of bulkhead framing to keep the shell from racking.

Although I didn't trailer it, the picture below is with interior skins only and framing for galley wall and front headboard cabinet. The shell was supported at three points. My wife and I crawled in - about 350 pounds of which I was the substantial majority - and the structure did not flex.



Adding a trailer to your shell will make the teardrop structure even stronger.

[Elmosaurus]

Hmmm.... I'm beginning to wonder if I should have planned in a galley just for the heck of it; as it stands now (no pun intended) there are no bulkheads in my design; I wanted maximum interior space for the bed and floor area.

The only bulkhead like supports I was going to have inside was the storage/bed supports that would be beneath part of the bed (about 10" high 3/4" plywood running the interior width on the floor) and a small cabinet over the bed area that was going to not really structural. (1/8" ply attached to a spar frame cubby hole cabinet)

I'm sure you guys would comprehend what I'm talking about better if I had actual drawn out plans to post pictures of... I've been working entirely in pencil and paper on little notepads. I suppose I could take a picture of them and post those...

Lemme see if I can whip out the master MS Paint skills really fast...

[Elmosaurus]

Ok, forgive the MS Paint... LOL. (if I had more time, I'd set up some CAD software and actually use it, but pencil and paper works faster for me)



The Black wireframe should give you a good idea of what the thing will look like.

The teal coloring is internal cabinets/bed supports. Probably all 3/4" ply. There will be cubby hole openings for simple access. The one up high on the rear wall will house a 22" tv probably.

The maroon color is the mattress. I'm aiming for the typical 'convert-a-dinette' that is found in many trailers. Center section will be a table top with cushion that is removeable, and can be installed on a pole in the center for instant 'booth' like dinette.

So, you can see, not really much in the way of bulkheads. Walls are 1x2 spar contruction, flat against the 3/8" plywood skin. (interior wall thickness of 3/4") All front, rear, and roof spars will be 1x2 spars, turned so that those walls are 1.5" thick (will have two layers of foam likely) with the .75" side of the spar glued and stapled to the exterior 3/8" ply.

Anybody think the thing will fold in half while driving down the interstate if it has no interior skins? (or, in general? )

E.

[planovet]

I think you will be ok. As long as you have a completed "box" and the exterior skins on it should be stable enough.

I built my sides complete (inside and out) before I put them on the trailer. Once I had the two sides and both bulkheads up it was very sturdy. It formed a very stout 4-sided "box". I drove it around the block with just 3 roof spars and no roof and it did well. Would not have taken it on the freeway though without a roof.

[Elmosaurus]

Ok, well, I guess we'll find out!

I'm sure the structure will hold together... (well, at least I hope. )

My real concern is will it cause enough flex to crack the primer and paint I put down; if there is enough flex to crack the finish, it could allow water intrusion at the seam joints of the 'box'.

I'll just have to go over every seam with a close inspection under halogen lighting after we get back, sand anything questionable and re-prime and re-paint...



E.

[parnold]

Could you build a temporary internal structure for additional support? Might put your mind more at ease!

[Elmosaurus]

Doing some more research since I haven't gotten to the yard to get more lumber just yet (I'm losing days here!... ugh) I found that most of the larger 21+ foot RV/Campers use 2"x2" spars for wall framing. (for those older units that used wood walls and not the newer composite sandwiched foam/wood/fiberglass walls)

I'm thinking it might be worth turning the spars such that the 1" (.75") edge is mounted to the walls; this will create a house wall like spar structure that should absolutely resist any kind of bowing. This will be especially useful since my interior plans don't really have much of any internal bulkheads, being an entirely 'open' floorplan for the most part.

Of course, it means I'm losing another 1.5" of interior space, but I think I'm ok with that at this point; it will add to the rigidity, give me more depth for wall and skin anchors, and allow for more space for insulation/internal wiring.

So, I guess I answered my own question!




Thanks for the help anyhow everyone!

E.

[Miriam C.]

E I used 1" (really 1" that I ripped from a 2x 8.) 1''x 1 1/2" and they are fine. Worked well for insulation too....I also cut the framing to allow the ceiling to be laid on top of the inside wall. The spars go in on top of the ceiling. Careful measuring....


I doubt if you need 3/4" for the bed. Some light framing makes a sturdy bed.
I also used 1/4" for the bed on top of a frame. One of the 1x3's is attached with a piano hinge and the others act as slats. There are 3 support sets. They are 2" x 4" s with
1" x 3"s going across. It works for us and we are not small.

[kennyrayandersen]

Hey guys, cut him some slack! It’s his first build – you know it’s going to be over-build and sturdy as a brick privy!

[Elmosaurus]

Thanks, Miriam.

I'm pretty comfortable now, having concluded a massive amount of searches and web research on older camper wood wall construction. Having seen what commercial campers use for wall construction, and their fastening techniques (long staples only, no glue!) I already know that since every wood joint is being glued, there will be much more rigidity and strength in our walls to begin with.

Since I'm also not handling the weight and dynamic flex loads of an 8' tall, 21'+ long wall, 1x2 framing should be more than fine for a 75" tall, 9.5' long structure. Especially since I'm using 3/8" ply exterior skin; most wood wall campers have aluminum as the outer skin and nothing but luan on the inside! Worse yet, they didn't do sandwich wall construction back then, they filled the cavities with fiberglass batting insulation. (I'll be gluing the foam into place against the exterior and interior skins, which should make the final wall extremely solid)

Knowing all this, and the fact that those older campers were on the road for decades and survived relatively fine, (with a lot more surface area to catch road wind blast) as I said, I'm pretty sure my TTT will hold up just fine now. And possibly be over built, like Kenny said! (although, I've been mindful the whole time to not get into that habit since it's mentioned so much around here)

Off to the yard for lumber today right after work!

E.

[aggie79]

The maroon color is the mattress.
E.

Maroon is good. Mark (Planovet) will agree with me.

[dh]

I think you will be ok. As long as you have a completed "box" and the exterior skins on it should be stable enough.

I built my sides complete (inside and out) before I put them on the trailer. Once I had the two sides and both bulkheads up it was very sturdy. It formed a very stout 4-sided "box". I drove it around the block with just 3 roof spars and no roof and it did well. Would not have taken it on the freeway though without a roof.

I would like to see that, a half built teardrop doing 90 down the freeway